The invention concerns porous fibers, methods for their preparation and for their use, in particular as carrier substances for active ingredients; and membranes in the form of flat foils, tube foils or hollow fibers of synthetic polymeric materials, methods for their preparation and for their use, in particular as filters or membrane carriers or supports for separation purposes.
Methods for the preparation of some types of porous fibers, i.e., fibers with porous structure, have long been known. In many instances, these fibers are referred to as foam fibers. The East German patent DDR-PS No. 103 375, for example, describes a process in which a polymer melt of one or a number of components, preferably incompatible components, is extruded with the addition of physical and/or chemical carrier material through one or a number of rows of spinning nozzle holes, which may be round, concave or concave-profiled. The thus-produced fibers can be worked up through the corresponding procedures into fleeces.
The prior art fibers with porous spaces exhibit numerous disadvantages. Some processes have therefore been developed which lead to fibers with pore spaces only in the interior; the surface is either completely closed or in any case exhibits a very limited porosity. Other types of fibers with openings in the surfaces in contrast have rough surfaces unsuitable for many uses.
There exists therefore a need for better methods of preparing such fibers, in particular methods which allow for simple preparation of extrudable spinning solutions and which obviate the need for complicated spinning baths. There is further the need for improved porous fibers which have even surfaces in spite of their having openings in the surface.
Similarly, membranes in the form of flat foils, tube foils or hollow fibers have been known to the art for quite some time. A number of methods are available to the person skilled in the art for preparation of such membranes from the most diverse polymers. Hollow fibers find use in the production of textile products; they also find use in filtration, ultrafiltration, microfiltration, dialysis, reverse osmosis, etc.
When used in separation devices, these membranes in the form of flat foils, tube foils or hollow fibers are selected on the basis of their permeability and selectivity, for the membranes should on the one hand be retentive of certain materials, and on the other hand, freely permeable to others, such as the solvent in a solution.
Foils with microporous or porous structure have been known for some time. So, for example, in DE-OS No. 27 37 745 a process for the preparation of foils of porous or microporous structure is described. The therein-described structures are without doubt valuable products which find use in a large number of fields.
In further investigation of the above teachings, however, it has been determined that the foils prepared thereby have a closed surface. In particular, it is not possible to obtain foils which have perceptible open pores on both surfaces. There remains therefore the need for improved methods for preparing porous foils characterized by good permeability coupled with high selectivity, and which in addition exhibit open pores on both surfaces while at the same time having flat even surfaces.
A method for the preparation of selectively permeable hollow fibers is described in DE-AS No. 14 94 579, in which an intimate mixture of a thermoplastic polymer with a plasticizer is melt spun and the plasticizer ultimately extracted from the thus-obtained hollow fiber. Thus, it is necessary that the plasticizer be easily and essentially completely extractable from the spun hollow fiber. Frequently this requires a comparatively long work time for complete extraction of the fiber, and it is not always possible to remove the plasticizer completely. Moreover, the hollow fibers prepared according to this method exhibit relatively low permeabilities. Finally, it is possible to vary the proportions of plasticizer and polymer within but a narrow range; at high plasticizer concentration, no fiber formation occurs; at low plasticizer concentrations, the permeability achieved is insufficient for most uses. There is further the danger that if the plasticizer is not sufficiently mixed with the thermoplastic polymer, there will be agglomerations which lead, after washing, to holes or oversize pores which preclude use of the hollow fibers for many purposes.
Another process for the preparation of hollow fibers is described in DE-AS No. 23 46 011, in which a solution of acrilonitrile copolymerizate is spun in an aqueous solution of mineral salts. For internal coagulation, it is necessary that coagulant be sprayed into the interior of the fiber. This process is comparatively complicated, and it is difficult to obtain hollow fibers with constant properties.
In U.S. Pat. No. 3,674,638 a process is described, in which first a solution of fiber-forming polymer is spun, then the exterior and generally the internal zones are treated to gel formation and simultaneously or subsequently coagulated. Thereby is obtained a hollow fiber which has a skin-type structure both on the inside and on the outside. This process is also comparatively complicated, and the permeability of these hollow fibers leaves much to be desired.
Thus, although many examples of membranes in the form of flat foils, tube foils and hollow fibers are known, there is always the need for improved membranes, in particular of the type which can be easily prepared from simple spinning solutions and without the need for complicated baths. There is further the need for improved membranes which are porous and which are characterized by good permeability coupled with a simultaneous high selectivity.